First, you will need to create a scrim team here and enter your team's details, such as the name of the roster and the password for new players to use to join the team. Your players should then join that team page, although, they can still do this once the team is entered in the ladder.
Next, head to the ladder listing and select the specific ladder that you want to join.
Once you have navigated to the specific ladder press "Join ladder" as so:
Finally, make sure the ladder and team page are correct before selecting to join the ladder: